This invention relates to digital phase control circuits and, in particular, to a phase control circuit which has improved transient response during its readjustment mode.
Digital phase control circuits are known which include a controllable oscillator, a programmable divider, a reference frequency source, a phase discriminator and a lowpass filter or integrating circuit. The output signal of the controllable oscillator is fed to one input of the phase discriminator via the programmable divider and the other input of the phase discriminator receives a signal from the reference frequency source. The low-pass filter circuit derives a control signal from the output of the phase discriminator so as to control the controllable oscillator.
The signals at the output of the phase discriminator have rectangular pulses. The average d.c. voltage of the rectangular pulses is obtained by means of the series-connected filter circuit which provides a setting voltage for the controllable oscillator. The circuit regulates itself in such a way that, in the steady-state, the signals applied to the phase discriminator coincide in frequency and phase.
In order to prevent excessive overshoot of the controllable oscillator, a minimum time constant is required in the filter circuit which may be designed, for example, as an active integrator. This results in a relatively long time constant for the entire system which can be detrimental in many cases. A long time constant may also increase the tendency toward resonance of the entire circuit.
It is an object of the present invention to provide a phase control circuit which is substantially improved with respect to its transient response during readjustment.